Photosynthesis
A cornfield in Jalisco grows from a few seeds into a plant two meters tall in just a few months. Where does all that matter come from, if the soil hardly loses any mass? The answer is in the leaves, and it is the basis of all the energy that flows through an ecosystem.
The overall equation and the pigments
Put a branch of an aquatic plant in lit water and you will see a steady stream of bubbles rise, which stops when the light is turned off. What gas is it, and where does the mass the plant gains come from?
$$6\,\mathrm{CO_2}+6\,\mathrm{H_2O}\xrightarrow{\text{luz}}\mathrm{C_6H_{12}O_6}+6\,\mathrm{O_2}$$In words: carbon dioxide plus water, with energy from light, produce glucose and oxygen. The reactants are CO₂ and H₂O, the products are glucose and O₂, and the energy source is sunlight.
Plants are not the only ones: the algae and phytoplankton of seas and lakes also carry out photosynthesis, and they produce a large part of the planet's oxygen.
Cellular respiration, the complementary process
Living things need usable energy. In cellular respiration, in both plants and animals, the molecules in food (such as glucose) are broken down and energy is released that the cell can use.
$$\mathrm{C_6H_{12}O_6}+6\,\mathrm{O_2}\rightarrow 6\,\mathrm{CO_2}+6\,\mathrm{H_2O}+\text{energía}$$Plants do both: they photosynthesize when there is light and respire all the time. Animals only respire. Together, photosynthesis and respiration are the two stages of the carbon cycle: the first takes carbon out of the atmosphere and puts it into organic molecules; the second returns it.
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Where | Chloroplasts | Mitochondria (and cytoplasm) |
| Reactants | CO₂ and H₂O | Glucose and O₂ |
| Products | Glucose and O₂ | CO₂, H₂O, and usable energy |
| Energy | Captured (light) and stored | Released |
| Who | Photosynthetic organisms | Almost all living things |
In autumn, in deciduous species, chlorophyll breaks down before the carotenoids, and the yellows and oranges that were previously hidden appear.
Write the overall chemical equation for photosynthesis and identify the reactants, products, and energy source of the process.
Write the overall chemical equation for photosynthesis and identify the reactants, products, and energy source of the process.
Which pigment captures light during photosynthesis and in which cell organelle is it located? Explain why leaves are green and what happens to this color in autumn in…
Which pigment captures light during photosynthesis and in which cell organelle is it located? Explain why leaves are green and what happens to this color in autumn in deciduous species.
Why photosynthesis matters
Thinking in terms of matter, energy, and atmosphere, photosynthesis is essential for three reasons:
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Matter: it converts inorganic carbon (CO₂) into organic matter. With it the plant is built and, when it is eaten, so are the herbivores and the entire food web. A plant's mass comes mostly from CO₂ and water, not from the soil.
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Energy: it transforms the energy of light into chemical energy stored in organic molecules. It is the energy input for almost all ecosystems.
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Atmosphere: it releases O₂, which we use to breathe, and removes CO₂, a greenhouse gas.
State and explain three reasons why photosynthesis is essential for life on Earth, considering matter, energy, and the atmosphere.
State and explain three reasons why photosynthesis is essential for life on Earth, considering matter, energy, and the atmosphere.
The chloroplast: thylakoids and stroma
Structure explains function. Photosynthesis takes place in chloroplasts, organelles of plant cells and algae. It is one of the "structures responsible" for capturing solar energy.
Distinguish between the light-dependent stage and the light-independent stage (Calvin cycle) of photosynthesis: in which part of the chloroplast each occurs, what…
Distinguish between the light-dependent stage and the light-independent stage (Calvin cycle) of photosynthesis: in which part of the chloroplast each occurs, what substances enter, and what is obtained in each stage.
From the leaf to the carbon cycle
Leaves exchange gases with the air through the stomata, small pores in the epidermis. CO₂ enters and O₂ exits through them (along with water vapor). Water arrives through the veins, and the glucose that is formed also travels through the veins to other parts of the plant.
Photosynthesis is the gateway of carbon into the biosphere: the carbon in atmospheric CO₂ passes into sugars, from there into the food web and, through respiration and decomposition, returns to the atmosphere. You will see this complete cycle in another section of the course.
The overall equations for photosynthesis and cellular respiration appear symmetrical. Reason why, despite this, they are not reverse processes: compare organelles,…
The overall equations for photosynthesis and cellular respiration appear symmetrical. Reason why, despite this, they are not reverse processes: compare organelles, mechanisms, and the net energy direction of each.
The diagram shows the exchange of matter in a leaf during photosynthesis. a) Indicate which substance is carried by the arrow marked "?" that exits through the vein and…
The diagram shows the exchange of matter in a leaf during photosynthesis. a) Indicate which substance is carried by the arrow marked "?" that exits through the vein and to which parts of the plant it travels. b) Write the overall equation for the process and relate each arrow to the reactants or products. c) Name the epidermal structure through which $\text{CO}_2$ enters.
Limiting factors
In a laboratory, as a plant is lit with ever more powerful lamps, the rate of photosynthesis rises at first and then levels off in a plateau. Why does more light stop speeding up the process?
The main factors are:
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Light: with little light, the rate grows almost in proportion to it; with a lot, it saturates (the capture machinery and the enzymes are already working at maximum).
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CO₂: it enters through the stomata. If the stomata close to save water, CO₂ becomes scarce.
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Temperature: the reactions depend on enzymes; in the cold they run slowly, and with excessive heat the enzymes are damaged.
This explains why productivity differs among ecosystems: a rainforest with plenty of light, warmth, and water photosynthesizes far more than a desert, where water and closed stomata limit the process.
A commercial tomato greenhouse in Almería installs supplemental LED lighting to increase winter production. When irradiance is increased from $400$ to…
A commercial tomato greenhouse in Almería installs supplemental LED lighting to increase winter production. When irradiance is increased from $400$ to $800\ \mu\text{mol}\times\text{m}^{-2}\times\text{s}^{-1}$, the $\text{CO}_2$ fixation rate only increases by $4\,\%$, reaching an unexpected plateau.
- a) Justify at the physiological and molecular level why, beyond a certain light intensity, the photosynthetic rate saturates even though more photons are available.
- b) Formulate a hypothesis regarding what alternative environmental factor is limiting the process in the greenhouse and design an experiment with controlled variables to test it.
- c) Reasonably predict how the light saturation curve and the maximum photosynthetic rate will change if the greenhouse atmosphere is enriched with gaseous $\text{CO}_2$.
In an agronomic trial in a semiarid area subjected to a heat wave ($38\ ^\circ\text{C}$) and summer drought, three adjacent plots are monitored: wheat (C3), sorghum…
In an agronomic trial in a semiarid area subjected to a heat wave ($38\ ^\circ\text{C}$) and summer drought, three adjacent plots are monitored: wheat (C3), sorghum (C4), and prickly pear (Opuntia, CAM).
- a) Vacuolar acidity (malate concentration) and stomatal opening are measured at 03:00 in the morning and at 14:00 at midday. Predict which crop will show maximum acidity at 03:00 and justify how this adaptation allows it to survive with minimal water expenditure.
- b) During the central hours of the day, wheat drastically reduces its net carbon assimilation rate to near-zero values, while sorghum maintains a very high positive net rate. Justify this contrast at the enzymatic level (RuBisCO versus PEP carboxylase) in relation to stomatal closure and photorespiration.
- c) In an experimental scenario with artificially enriched $\text{CO}_2$ but without water deficit, predict which of the two crops (C3 or C4) will experience a greater percentage increase in its photosynthetic rate and reason why.
Investigating with data
To find out how a factor has an influence, you have to change only one variable, measure the result, and keep everything else constant. These are simulated data of net O₂ exchange (arbitrary units per minute) of an aquatic plant at constant temperature and CO₂:
| Relative light | Replicate 1 | Replicate 2 | Replicate 3 | Mean |
|---|---|---|---|---|
| 0 | −2 | −1 | −3 | −2 |
| 1 | 3 | 4 | 5 | 4 |
| 2 | 7 | 8 | 9 | 8 |
| 3 | 8 | 9 | 10 | 9 |
To reforest a degraded hillside, a sapling with an initial dry mass of $50\ \text{g}$ is planted. After two years, its dry biomass (primarily made up of cellulose, a…
To reforest a degraded hillside, a sapling with an initial dry mass of $50\ \text{g}$ is planted. After two years, its dry biomass (primarily made up of cellulose, a polymer of glucose) reaches $2.5\ \text{kg}$, while the surrounding soil has only lost $15\ \text{g}$ of net mineral salts.
- a) Refute the common misconception that plant mass comes from the soil: justify the origin of the $2450\ \text{g}$ of gained organic matter and calculate what minimum mass of atmospheric $\text{CO}_2$ the plant must have assimilated to synthesize that equivalent amount of glucose ($M_m(\text{glucose}) = 180\ \text{g/mol}$; $M_m(\text{CO}_2) = 44\ \text{g/mol}$).
- b) Measurements with gas analyzers indicate that the plant actually fixed a significantly higher mass of $\text{CO}_2$ than that calculated in the previous section. Biologically explain this discrepancy based on the balance between gross photosynthesis, net photosynthesis, and cellular respiration.
- c) If the tree were watered with water labeled with the heavy isotope $^{18}\text{O}$ ($\text{H}_2^{18}\text{O}$) and supplied with ordinary $\text{CO}_2$, predict with justification in which of the photosynthesis products (in glucose or in the emitted $\text{O}_2$) this isotope will be detected, detailing the photosynthetic phase involved.